Atmospheric river set to impact the West Coast with heavy rainfall
A powerful atmospheric river (AR) is expected to bring significant rainfall to the West Coast, particularly affecting Washington and Oregon starting October 25, 2024, and continuing into early October 27. Following this, a second AR may impact the California-Mexico border and parts of the southwestern U.S. on October 28 – 29.
Forecast of two atmospheric rivers (AR) set to make landfall on the U.S. West Coast between October 25 - 29, 2024, with heavy rainfall probabilities depicted on the left and AR scale intensity on the right. Image credit: CW3E
- The AR will transport moisture from the tropics, enhancing precipitation levels in coastal regions and extending inland into areas like Colorado and New Mexico.
- The first atmospheric river is expected to deliver 50 – 127 mm (2 – 5 inches) of rain in the Pacific Northwest, while lighter amounts of 12.7 – 38.1 mm (0.5 – 1.5 inches) are forecast for Northern California.
- The second atmospheric river event will bring additional precipitation to California’s southwest and extend into the Upper Colorado River Basin, southwestern Colorado, and northern New Mexico, with inland areas expecting 12.7 – 38.1 mm (0.5 – 1.5 inches) of rain.
A powerful atmospheric river (AR), a long, narrow zone in the atmosphere that transports water vapor from the tropics, is likely to bring significant rainfall to the West Coast, with moderate AR conditions starting today, October 25.
The CW3E’s AR landfall tool, powered by the Global Ensemble Forecast System (GEFS), shows a high probability (over 90%) of moderate AR conditions impacting Washington and Oregon during the first AR.
The first AR is expected to continue into early Sunday, October 27, delivering 50 – 127 mm (2 – 5 inches) of rain across the Olympic Peninsula, Oregon Coast Ranges, and the Washington/Oregon Cascades.
In contrast, lighter rainfall of approximately 12.7 – 38.1 mm (0.5 – 1.5 inches) is forecasted for the Northern California Coast Ranges and Northern Sierra Nevada.
This initial AR system is driven by strong southwesterly winds that are expected to prolong the rainfall in the region, intensifying moisture flow.
A second AR system is forecasted to make landfall near the California-Mexico border between October 28–29, carrying moisture from Tropical Cyclone “Kristy” as it interacts with an onshore trough.
This second event will bring additional precipitation to parts of California’s southwest and extend into the Upper Colorado River Basin, southwestern Colorado, and northern New Mexico.
Inland regions could see up to 12.7 – 38.1 mm (0.5 – 1.5 inches) of precipitation as the AR penetrates the western U.S.
In addition to rainfall forecasts, Integrated Vapor Transport (IVT) values, a measure of the intensity of water vapor flow, indicate significant moisture transport with both upcoming atmospheric river systems.
The initial AR IVT values are expected to exceed 500 mm (19 inches) over parts of Washington and Oregon. This suggests moderate AR conditions with substantial moisture flux along coastal regions, especially over the Olympic Peninsula and Cascades.
IVT values will remain elevated during the second AR event, likely over 250 mm (9.8 inches) in inland areas like southwestern Colorado and northern New Mexico.
These high IVT measurements stress the atmospheric river’s potential to bring moisture well beyond the coast, potentially sustaining precipitation in inland areas even after landfall.
Hydrologic impacts are currently not expected to be severe, but winter storm conditions could develop in Colorado, stressing the potential for a wide range of weather effects across the region.
This forecast, issued by the Center for Western Weather and Water Extremes (CW3E) and the National Weather Service (NWS), raises concerns for the Pacific Northwest and Northern California areas.
References:
1 Quick Look at Atmospheric River Forecast to Make Landfall over Pacific Northwest – CW3E – October 24, 2024
Harsha Borah is an experienced content writer with a proven track record in the industry. Harsha has worked with LitSpark Solutions and Whateveryourdose, honing skills in creating engaging content across various platforms. A gold medalist in a state-level writing competition organized by Assam Tourism, Harsha’s travelogue on Tezpur was widely appreciated. Harsha’s article, "The Dark Tale of the Only Judge in India to Be Hanged," ranks second on Google and has garnered over 11 000 views and 8 900 reads on Medium. Outside of writing, Harsha enjoys reading books and solving jigsaw puzzles.
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